The Communist party will not field candidates in the general election for first time in almost a century after suggesting Jeremy Corbyn will help achieve its "revolutionary" aims.
The party's general secretary said communists would provide support on the ground for Mr Corbyn, who is a long-term supporter of the movement and has previously called Fidel Castro "heroic".
Announcing the historic decision yesterday Robert Griffiths said: "The maximum possible Labour vote is necessary not only to secure the election of a Labour government.
"The higher the Labour vote and the number of Labour MPs elected, the more secure will be the position of Jeremy Corbyn and his left allies in the Parliamentary Labour Party."
The party vowed to produce its own manifesto "in order to project our revolutionary strategy for socialism and a programme of clear, consistent policies for the labour movement" as it pledged support for Mr Corbyn's aims.
He added that any losses suffered under Mr Corbyn would be "used as a pretext by the right-wing pro-EU, pro-NATO faction in the Parliamentary Labour Party and its trade union allies to launch yet another bid to remove Jeremy Corbyn and take the Labour Party back to the neoliberal and pro-war policies of the past".
Mr Griffiths's party, which won just over 1,000 votes at the last election, stood nine candidates in 2015 and has fought for votes at every election since the 1920s.
Tory MP Amanda Milling, a member of the Business, Energy and Industrial Strategy Select Committee said: "Let this sink in: Jeremy Corbyn has been endorsed today by a party who want to close down businesses, leave the protection of NATO and hike up taxes to a level never seen before in this country.
"At this election every vote is going to count. Unless people get out there and vote for Theresa May and their local Conservative candidate, this man will be our Prime Minister in a little over 6 weeks – propped up by the Lib Dems and SNP in a coalition of chaos."
The endorsement came as it emerged that Andrew Fisher, the man writing Labour's 2017 election manifesto, previously called for the police to be disarmed, M15 disbanded and defence spending to be slashed.
The policies were part of a proposal by the Labour Representation Committee in 2013 when Mr Fisher was joint secretary and a Socialist Campaign for a Labour Victory pledge he signed two years later.
Conservative MP James Cleverly said: “All you need to know about the man writing Labour’s manifesto is that he wants to make our country less safe."
Yesterday Mr Corbyn vowed to give even stronger support to trade unions and called the movement "heroes" in a speech in Scotland.
The Labour leader promised to immediately abolish the Trade Union Act if elected in June as he offered a series of other pledges to the movement.
He also ruled out an electoral pact with the Scottish National party.
He said: "The Labour party will always cherish, sustain and protect our relationship with the trade union movement and the working people you represent.
"You are our DNA, you are our family, and we will never, ever apologise for the closeness of our relationship with you.”
But his remarks prompted Conservatives to warn there would be more strikes under a Labour Government and that people would be less safe.
Priti Patel, the International Development Secretary, said: “Let’s be clear what Jeremy Corbyn is calling for today. He’s calling for it be easier to go on strike in our emergency services and border posts.
"He’s calling to remove protections on crowd control at pickets. And he’s calling for union bosses who bankroll Labour to be allowed to keep their campaign spending a secret.
"What’s more, the Lib Dems, SNP and Greens would make it happen. “Unless people turn out and vote Conservative for the strong stable leadership of Theresa May - Jeremy Corbyn will be Prime Minister at the head of a coalition of chaos.
"That would throw away all of the economic progress we’ve made – and have grave consequences for our Brexit negotiations.”
In a day of further turmoil for the party yesterday Andrew Gwynne, Labour's election campaign chief, contradicted his leader and said he would kill ISIL leader Abu Bakr al-Baghdadi if offered the chance.
On Sunday Mr Corbyn refused to answer the question, but Mr Gwynne clarified: "Yes ... The Labour Party will never ever put the defence of the realm at the back of the queue."
It came as Nia Griffiths, the party's shadow defence secretary, also contradicted Mr Corbyn and promised that Trident renewal will be included in Labour's manifesto after the Labour leader hinted it might not be on Sunday.
Later in the day Tom Watson, the party's deputy leader, suggested Labour will protect the pension triple lock in a bid to stave off Ukip in northern heartlands.
He told The Yorkshire Post: "The characteristic of the Ukip voter is they are a little bit older. You now actually have a dividing line for people who are on state pensions where the Government have said they are not committed to David Cameron’s triple-lock on pensions and John McDonnell and Jeremy Corbyn have been emphatic they are going to retain that.
“So there is a very distinct pensioner offer from Labour in this election that perhaps you could say we haven’t emphasized in the 2015 election."